An aliphatic polyamide typified by nylon 6 or nylon 66 has excellent properties such as heat resistance, chemical resistance, rigidity, abrasion resistance, and moldability, and hence is used for a variety of applications as an engineering plastic. However, the aliphatic polyamide is known to have problems such as heat resistance in an application such as an automotive part where the polyamide is exposed to a high temperature, and low dimensional stability due to water absorption. In particular, in recent years, demands on the heat resistance tend to increase in an electrical and electronic part application using a surface mount technology and in an automotive part application such as an electrical component part in an engine room, and it is difficult to use the conventional aliphatic polyamide. Therefore, it has been desired to develop a polyamide excellent in heat resistance, dimensional stability, and mechanical properties.
In addition, the aliphatic polyamide not only has excellent abrasion resistance but also hardly causes burning even in an unlubricated state. Further, the aliphatic polyamide generates little noises and is excellent in lightweight property and corrosion resistance as well, and hence is often used in sliding parts such as a bearing, a gear, a bush, a spacer, a roller, and a cam. On the other hand, in the case where the conventional aliphatic polyamide is used under a severe condition where high friction is continuously generated, an increased temperature due to frictional heat causes melting as well as significant abrasion, which makes it difficult to continue a steady frictional motion. Moreover, the conventional aliphatic polyamide undergoes a dimensional change due to water absorption and shows reductions in mechanical properties, and it is required to improve the problems.
In order to meet such demands, there is used, as an engineering plastic, a semi-aromatic polyamide called 6T polyamide, which has a melting point higher than that of the conventional polyamide and contains a polyamide formed of 1,6-hexanediamine and terephthalic acid as a major component (see, for example, Patent Document 1). However, the polyamide formed of 1,6-hexanediamine and terephthalic acid has a melting point of about 370° C., and hence cannot be used actually because melt molding needs to be carried out at a temperature equal to or higher than a polymer degradation temperature. Therefore, in actual use, adipic acid, isophthalic acid, ϵ-caprolactam, or the like is copolymerized at about 30 to 40 mol % to prepare a polyamide having a composition to achieve a melting point as low as about 280 to 320° C. which is a temperature range that enables actual use of the polyamide.
Such copolymerization of a third component or a fourth component is effective for lowering the melting point, but may lead to lowering of a crystallization rate and a final crystallization degree. As a result, not only physical properties such as rigidity, chemical resistance, and dimensional stability at a high temperature are lowered, but also productivity may be lowered due to elongation of a molding cycle. Further, the polyamide has a drawback in moldability because the viscosity is easily lowered in melt retention. In order to solve such problems, it has been proposed that the problems are improved by blending a filler such as a glass fiber, a carbon fiber, a glass powder, or a graphite powder in the polymer (see, for example, Patent Documents 2 and 3). This can solve the problems to some extent, but physical properties such as the rigidity, chemical resistance, and dimensional stability are insufficient in some cases.
As a high melting point polyamide other than the 6T polyamide, there has been proposed a semi-aromatic polyamide, which is formed of a mixture of 1,9-nonanediamine and 2-methyl-1,8-octanediamine and terephthalic acid and is called 9T polyamide (see, for example, Patent Document 4). The 9T polyamide has a higher crystallization rate, a higher final crystallization degree, and lower water absorbability compared with the 6T polyamide which is the semi-aromatic polyamide and is put to practical use. However, as is the case with the above-mentioned problems, it was impossible to solve problems such as lowered physical properties due to copolymerization and lowered melt flowability due to the aromatic dicarboxylic acid used as a major component.